The European Union has appealed to Serbia and Kosovo for restraint amid escalating tensions along their northern border.

EU foreign policy chief Catherine Ashton on Friday urged the two sides to work toward a return to dialogue. Ashton said an EU envoy will visit the region in the coming days to attempt to get the two sides back to the negotiating table.

Ethnic Serbs from Kosovo's north said on Friday they had blockaded NATO troops, who took control of border posts with Serbia this week.

NATO moved in after Kosovo's ethnic-Albanian government sent special police units Monday to take control of northern border crossings to enforce a ban on imports from Serbia. The ban was in retaliation for Serbia's blocking of Kosovo's exports. One policeman was killed and several other people were wounded in an exchange of gunfire between the two sides.

After Kosovo police withdrew Wednesday under an agreement negotiated by NATO, about 200 mostly masked Serbs armed with firebombs attacked the border posts.

The Serb enclave has refused to recognize the government's authority since Kosovo seceded from Serbia in 2008. Belgrade also rejects that secession.